1. Field of the Invention
The present invention relates generally to the Global Positioning System (GPS) and more particularly to GPS reception in an interference environment.
2. Description of the Related Art
Numerous guidance and navigation systems benefit from use of the GPS which provides a wealth of information such as latitude, longitude, height, velocity and the exact time. The GPS constellation contains 24 satellites which are distributed throughout 6 different orbits. GPS signals are at carrier frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz) and use spread spectrum signals with 1.023 Mbps binary phase shift keying (BPSK) modulation for a "short" coarse/acquisition (C/A) code and 10.23 Mbps modulation for a "long" precision (P) code.
With right-hand circular polarization, each GPS satellite transmits the following three signals: the C/A code modulated on the L1 carrier frequency with a 1 millisecond repetition rate, the P code modulated on the L1 carrier with a weekly repetition period and the P code modulated on the L2 carrier with a weekly repetition period.
The GPS constellation's design insures that 6 to 11 satellites are in view from any point on the earth's surface at any given time. Because of the GPS signal design, two-dimensional and three-dimensional positions can be determined with the signals from just three and four satellites respectively. Accordingly, GPS receivers typically have the capability of automatically selecting three or four of the satellites in view based upon their received signal strength and Position Dilution of Precision (PDOP).
A number of undesirable interference sources (e.g., deliberate electronic countermeasures, RF electromagnetic pollution, clutter scatter returns and nature noise) can cause a GPS receiver to be ineffective or unreliable. Receiver failure is generally due to missing synchronization in the spread-spectrum Pseudo Random Noise (PRN) code.
Accordingly, adaptive antenna arrays have been proposed in which knowledge of each element's received signal is used to modify those signals with weights (e.g., phase weights) that generate a null in the interfering signal's direction.
Various adaptive array systems are based on gradient-based algorithms which estimate the antenna's output-power gradient. Because processed signals from each element of the array are typically required for gradient computation, these receiving systems are often said to use a "multiport technique". In order to compute gradient of power, or correlation or other performance, the signal from every element must be accessible in these systems, i.e., a separate receiving channel is required for every element in the antenna array. Multiport techniques therefore require a separate coherent receiver channel for each element in the antenna array.
This is illustrated in FIG. 1 which shows an exemplary multiport structure 20 that has an array 22 of N antenna elements 24. Each element 24 feeds a different receiver channel 26 that includes an adjustable weighting element (e.g., a phase shifter) 28, a low noise amplifier 30, a down converter 32 for down converting the signal frequency from L band to baseband (or IF band) and a pair of analog-to-digital converters (ADC) 34 for partitioning the downconverted signal into I/Q components.
The digitized I/Q signals are coupled to a digital signal processor (DSP) 36 which performs optimal weight computations that require up to n(n+1) auto-correlation and cross-correlations and an n.times.n inverse matrix computation. A weight controller 38 then feeds the computed phase shifts back to the phase shifters 28.
An exemplary multiport technique is described in U.S. Pat. No. 5,694,416 (issued Dec. 2, 1997 to Russell K. Johnson) and another is shown in U.S. Pat. No. 5,471,220 (issued on Nov. 28, 1995 to David E. Hammers et al.) which illustrates a microwave packaging scheme that includes a coplanar section, fiber optical network, a plurality of adaptive beam processors, a fiber optical network, one or more signal processing modules and a set of microprocessors. In the coplanar section, a "sandwich style" package includes 3 layers: an antenna layer consisting of a plurality of elements, a transceiver layer consisting of a plurality of transceivers, and a beam forming layer.
U.S. Pat. No. 5,712,641 (issued on Jan. 27, 1998, to Mario M. Casabona et al.) describes an adaptive cross polarization interference cancellation system for GPS signals. It is based on dissimilarity between the right hand circular polarization of the GPS signal and the polarization of the interference signals. An orthogonally-polarized antenna system decomposes the received GPS signal into vertical-polarization and horizontal-polarization signals. Both are fed to an adaptive antenna feeding system which is controlled by an interference detection circuit and the resultant cross polarization attenuates the interference signals.
A reception technique which utilizes an analog to digital converter (ADC) prior to GPS signal processing is described in U.S. Pat. No. 5,347,284 (issued on Sep. 13, 1994, to John P. Volpi et al.). The ADC uses 4 level coding and full zone processing. A threshed detector senses the difference of a probability density function (PDF) in time distribution between the GPS signals' spread spectrum and continuous wave (CW) signals. For the CW signal, The PDF of the CW signal has a saddle shape (a falling off between two peak ending values) whereas that of the GPS signal is nearly uniform. Attenuating the digitized data in the vicinity of the peak ending values provides a degree of immunity to CW interference.
Although these conventional reception methods may improve reception of GPS signals in interference environments, they typically are hardware intensive (e.g., U.S. Pat. Nos. 5,694,416, 5,471,220 and 5,712,641) or limited to particular interference signals (e.g., U.S. Pat. No. 5,347,284).